The present invention relates to a method for operating a multi-cylinder combustion engine with variable compression ratio. Moreover, it also relates to a control device which controls at least the compression of a multi-cylinder combustion engine.
A method and a control device of this type are disclosed in the German patent document DE 100 51 271. The reference discloses a combustion engine with a crankshaft which is not directly supported in a motor block. Instead, the crankshaft is supported in eccentric rings, which in turn are supported rotatably in supporting bearings in the motor block. By means of an adjusting mechanics, the eccentric rings can be turned in a controlled way. During turning of the eccentric rings, the position of the crankshaft relative to the motor block changes. While the cylinders of the combustion engine are connected with the motor block, the pistons of the combustion engine which are movably guided in the cylinders are connected with the crankshaft through connecting rods of constant lengths. A change of the position of the crankshaft relative to the motor block therefore results in a change of the position of the pistons in the cylinders of the combustion engine. In particular, also the position of the upper dead point of the pistons in the cylinders changes. As a result, also the compression volumes enclosed by the pistons in their upper dead point position change. Since the lower dead point position of the piston changes in the same way as the upper dead point position, the stroke volume VH of the combustion chamber does not change with a change of the crankshaft position relative to the motor block. The change of the compression volume VC with constant stroke volumes VH implies a change of the compression ratio ε=(VH+VC)/VC.
Alternatively to this adjusting mechanics, in which the distance between the crankshaft position and the cylinder is controllably changed, also systems are known in which the compression ratio is changed by a tilting of the motor block relative to the crankshaft position, or by a tilting of the cylinder head relative to the motor block, or by lifting or lowering of the cylinder head relative to the motor block in a controlled manner. All these methods have in common that the geometrical compression ratio ε=(VC+VH)/VC can be changed by a controlled change of the compression volume VC.
In contrast to conventional combustion engines, having a compression ratio ε which is determined by the geometry of the combustion chamber and is fixed, the thermodynamic efficiency of the combustion engine in partial load region can be increased with a variable compression. As a result consumer advantages are provided. This is connected with a reduction of the CO2 emissions. The higher the compression ratio, the higher the compression end temperature. Since with increasing compression and temperature there is a danger that the combustion chamber filling in an Otto engine ignites itself (knocking), the maximum possible combustion ratio is limited by the tendency to knocking of the fuel. A reliable direct start and a faster motor high running is possible in start-stop operation.
In conventional combustion engines with fixed compression ratio the maximum compression ratio is structurally determined so that with the maximal combustion chamber filling (full load) no knocking occurs. It follows from this that with structurally fixedly predetermined combustion ratio and combustion chamber fillings under maximum possible value (partial load) critical compression and temperatures substantially are not reached. The efficiency of the combustion remains then behind an optimal efficiency. With the variable combustion this efficiency loss can be counteracted. Conventionally, the geometric compression ratio of a combustion engine with variable compression increases with increasing load (combustion chamber filling).
Independently from the above described concept, variable compression provides phenomena, that a direct start of a combustion engine must be possible with significantly avoiding of the use of electrostarters. In connection with this, the German patent document DE 101 11 928 A1 discloses that before a start, the position of a piston in a cylinder of the internal combustion engine is to be determined. In this way, the cylinder of the combustion engine is identified, whose piston is located in a working stroke. Then, fuel is injected directly into combustion chamber of the cylinder, and the combustion chamber filling is ignited directly after the injection, for releasing a self start or direct start of the combustion engine.
When this self starting concept is transferred to combustion engines with variable compression, worsened starting properties are produced than in the case of direct start concept in connection with constant compression ratio.